Ionic covalent organic frameworks-based electrolyte enables fast Na-ion diffusion towards quasi-solid-state sodium batteries

Solid-state sodium batteries present a high potential for future energy technology due to their high safety and energy density. However, sluggish Na⁺ transportation of solid-state electrolytes and serious Na dendrites hinder their further development. Herein, we propose a negatively charged-modified covalent organic framework (COF) with -SO₃Na as a Na-ion quasi-solid-state electrolyte (QSSE-COF-SO₃Na) for the first time to enhance the Na⁺ transportation. Density functional theory calculations and molecular dynamics simulations prove that the nano-scale ion channels of the COF-SO₃Na and the interaction between the -SO₃^- and the anion PF₆^- effectively enhance the Na⁺ diffusion kinetics. The QSSE-COF-SO₃Na exhibits a high ionic conductivity of 4.1 × 10^–4 S cm^-1 at room temperature and a high transference number of 0.89. Particularly, Na|QSSE-COF-SO₃Na|Na symmetric cells show a stable Na plating/stripping process without Na dendrites over 1000 h and 800 h at 0.05 and 0.2 mA cm^-2, respectively. Additionally, the QSSE-COF-SO₃Na supports full cells, which respectively use NaTi₂(PO₄)₃, Na₃V₂(PO₄)₃, and NaFePO₄ as cathodes, to display good cycling stability and rate performance. This work highlights the novel strategy to develop the Na-ion quasi-solid-state devices.